The present invention is directed generally to combustion apparatus or boilers utilized for the extraction of energy values from hydrocarbon fuels such as coal or from hydrocarbon fuels contained in oil shales and similar-type containments, and more particularly, to a combustor in the configuration of a continuous loop for providing constant recycling of unburned fuel particulates for effecting the efficient extraction of energy values from the fuel.
The extraction of heat energy from hydrocarbon fuels such as coal, has long been a standard. However, with the ever increasing cost of such fuels and the impact on the environment caused by the burning of such fuels, considerable changes in the various combustion systems were required to meet demands for an efficient energy extraction system which will satisfy environmental standards. The conventional boiler systems such as pressurized combustors, are somewhat inefficient in that considerable quantities of combustible organic material is discharged with the ash and flue gas. Further, the sulfur compounds and nitrogen oxides escaping with the flue gas have had a considerable impact upon the environment.
Efforts to provide an efficient combustion system which will satisfy environmental standards has been substantially satisfied by the use of the fluidized bed combustion system. Such a system provides for efficient fuel combustion, heat transfer and desulfurization in a single reactor which effectively eliminates desulfurization and nitrogen oxide removal units required of previous combustion systems. In a fluidized bed reactor, the combustion air passes through a fluidized bed of coal, ash particles, and sulfur sorbent material such as limestone or dolomite, at various velocities to provide the fluidized turbulent motion of these particles within the fluidized bed. The turbulent motion results in efficient combustion of the combustible organic material and provides for the continuous stirring of the sorbents to effectively reduce the sulfur dioxide concentration in the flue gas.
While the fluidized bed combustor has provided a considerable advancement in the combustion art, several shortcomings are still present with the fluidized bed combustion system which detract from the overall efficiency thereof. For example, in a fluidized bed combustor, a high freeboard area is required for effecting essentially complete combustion of combustible organic material in the freeboard. However, because of this height requirement, a substantial temperature variation exists in the freeboard such that some nitrogen oxides are discharged from the system. Further, even with a high freeboard area, there is still a relatively high percentage of the combustible organic material which escapes with the flue gas. The elutriation of unburned carbon fines in the flue gas has been a major contribution to poor combustion efficiency. Elaborate afterburner provisions such as a carbon burn-up cell and flyash reinjection have been used to minimize this problem. However, the complexity of these afterburner provisions has increased the difficulties of combustion operation.